A radial-anisotropic magnet and a polar-anisotropic magnet shown in FIGS. 15(a) and 15(b) are known as magnets used in rotors, etc. of permanent magnet synchronous motors. Either magnet generates a cogging torque when installed in a motor. The cogging torque depends on the rectangularity of magnetic poles and an attraction force between magnetic poles. Because a larger cogging torque results in larger vibration and noise while a motor is rotating, it is desirable to reduce the cogging torque. Various attempts have been made to reduce the cogging torque; (a) contriving a stator tooth tip design, (b) providing a magnet with a magnetization waveform close to a sine wave, (c) optimizing the number of magnetic poles on both a stator and a rotor, (d) skewing or skew-magnetizing a rotor to provide it with magnetization averaged in a rotation axis direction, (e) widening gaps between magnetic poles, etc.
Among them, the means (b) of providing a magnet with a magnetization waveform close to a sine wave can reduce the cogging torque without decreasing its power. Because a magnetic flux between adjacent magnetic poles flows in an arcuate form in a polar-anisotropic ring magnet, a surface magnetic flux density distribution has a waveform close to a sine wave, so that a polar-anisotropic ring magnet is advantageous over a radial-anisotropic magnet having a surface magnetic flux density distribution in a rectangular waveform. In addition, because a polar-anisotropic ring magnet can have a surface magnetic flux density as high as about 1.5 times that of a radial-anisotropic ring magnet, higher power can be obtained by the polar-anisotropic ring magnet than by the radial-anisotropic magnet.
As an example of an apparatus for molding a polar-anisotropic ring magnet, JP 2000-269062 A discloses a molding apparatus used in a magnetic-field-molding step in the production of an anisotropic ring magnet having a plurality of poles on its surface by a sintering method, which comprises a cylindrical, nonmagnetic die and a columnar rod core for defining a ring-shaped, molding space, the molding die having n grooves to produce a magnet having n magnetic poles, a coil for applying a magnetic field being received in each groove, and high-magnetic-permeability members being buried between adjacent grooves of the die However, this molding apparatus fails to provide a polar-anisotropic ring magnet with a surface magnetic flux density distribution sufficiently close to a sine wave.
To make the surface magnetic flux density distribution of a polar-anisotropic ring magnet close to a sine wave, namely, to make it have a desired sine wave-matching rate, magnetic powder should be properly oriented in magnetic field molding. However, JP 2000-269062 A does not teach at all in which direction the magnetic powder should be oriented to make its surface magnetic flux density close to a sine wave. With such factors as sintering deformation, etc., the sine wave-matching rate of the surface magnetic flux density distribution is about 85 to 90% in mass-produced magnets.